Stilbene bond rotation energies

The above discussion is applied to most of the arylethenes. The importance of the single-bond rotation was argued from the view point of nonvertical excitation transfer from the low-energy sensitizer to the cw-stilbene [33-36,39, 112,113]. In the case of 8a, even for the trans Isomer the dihedral angle between the pyrenyl plane and the double bond is estimated to be about 30° due to the steric effect of the hydrogen atom at the peri position. In contrast, the dihedral... 

The common feature of the ethene, styrene, and stilbene S Sq transformations is that double-bond isomerization involves not only twisting at the double bond but also pyramidalization. The pyramidalization modifies the energies that are depicted in Figure 12.10, which considers only the bond rotation, and lowers the energy of the state sufficiently to permit rapid internal conversion. 

Theoretical calculations have been made on stilbene which are relevant to photoisomerization dynamics. MNDO calculations of stilbene potential energy properties shows no evidence of a doubly excited "phantom" state but a singly excited state with adiabatic rotation around the central ethylene bond has only a small barrier on this path23T Calculations of dipole moments, optical spectra, and second order hyperpolarizability coefficients of some mono- and disubstituted stilbene molecules allows the design of useful nonlinear optical molecules 38. 

The cis-trans isomerization of stilbene (4) has been thoroughly studied. The potential-energy diagram for a rotation about the CC double bond that preserves a twofold symmetry axis is shown in Figure 7.3. 

Stilbene is a molecule which exists in two isomeric states because of the large energy barrier associated with rotation around the central double bond (see fig. 7.13). It has been known for some time [22] that when stilbene is excited to its first singlet state by absorption of UV light, it rapidly fluoresces to the ground state but with some molecules in the other isomeric form. Thus, if c -stilbene is irradiated with ultraviolet light, one obtains approximately 50% tran -stilbene and... 

Olson was the first to postulate that optical excitation of the ethylenic double bond involves rotation around a double bond in its excited state and that this rotation leads to an observable photoisomerization process [8-10]. Olson dealt with this aspect in terms of potential energy curves and mentioned the possibility of adiabatic photoisomerization process. Later, Lewis and co-workers [11] studied the photoisomerization process of tronj-stilbene with great interest but could not detect the cw-stilbene fluorescence. More recently, more detailed fluorescence studies carried out by Saltiel and co-workers [12-15] revealed that cw-stilbene fluoresces very weakly (Oa 0.(X)01) and shows an inefficient adiabatic isomerization process. The singlet mechanism currently proposed by Saltiel [16] is supported by quenching studies [17-20]. The extensive studies carried out on stil-bene and its analogs have already been reviewed [21-23]. Here the nature of the singlet excited state involved in the trans-cis isomerization process is dealt with. 

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